In the silicon wafer the density of interstitial oxygen (hereinafter termed “Oi”) is one very important product characteristic for determining the performance of a semiconductor device. The Oi density is determined during the pulling up process for a silicon single crystal according to the CZ method. In the crystal pulling up process, the rotational speeds of the crucible and the crystal, and the flow rate of an inactive gas such as argon or the like within the chamber and so on, are used as manufacturing parameters. However there is the problem that, with only these parameters, there is a limit to the range over which it is possible to control the Oi density, so that the range for good product quality which matches the Oi spec requested from the side of the device manufacturer is narrow (in other words, the yield rate is bad).
In order to ameliorate this problem, techniques are known for enhancing the yield rate by providing multiple heaters, arranged along the vertical direction within the chamber (see Patent References #1 and #2). In Patent Reference #1, it is disclosed to widen the control range for the Oi density, and to improve the yield rate, by changing the power supplied to each of the multiple heaters according to the pulling up length of the crystal. And, in Patent Reference #2, it is disclosed to enhance the controllability of the Oi density by changing the power supplied to each of the multiple heaters along a curve which is specified according to the pulling up rate of the crystal.
Patent Reference #1: Japanese Laid-Open Patent Publication Showa 62-153191;
Patent Reference #2: Patent Publication 2,681,115.
According to these prior art techniques, the Oi control range is widened by using a multiple heaters. However, the obverse of this is that the controllability of the crystal diameter becomes bad, and the actual diameter sometimes becomes excessively great as compared to the required diameter and sometimes becomes excessively small; and, as a result, the product yield rate becomes bad. Thus, it is not possible to anticipate enhancement of the yield rate by using a multiple heater in this manner.